The present invention relates to electrostatic dissipative thermoplastic urethanes (TPU) and compositions thereof.
The formation and retention of charges of static electricity on the surface of most plastics is well known. Many plastic materials have a significant tendency to accumulate static electrical charges due to low electrical conductivity. This type of formation and retention of charges of static electricity can be problematic. The presence of static electrical charges on sheets of thermoplastic film, for example, can cause the sheets to adhere to one another thus making their separation for further processing more difficult. Moreover, the presence of static electrical charges causes dust to adhere to items packaged in a plastic bag for example, which may negate any sales appeal.
The increasing complexity and sensitivity of microelectronic devices makes the control of static discharge of particular concern to the electronics industry. Even a low voltage discharge can cause severe damage to sensitive devices. The need to control static charge buildup and dissipation often requires the entire assembly environment for these devices to be constructed of partially conductive materials and/or electrostatic discharge (ESD) materials. It also may require that electrostatic protective packages, tote boxes, casings, and covers be made from conductive polymeric materials to store, ship, protect, or support electrical devices and equipment.
The prevention of the buildup of static electrical charges which accumulate on plastics during manufacture or use has been accomplished by the use of various ESD additives such as antistatic agents and ESD ingredients. These additives can be applied as a coating which may be sprayed or dip coated on the article after manufacture, although this method usually results in a temporary solution. Alternatively, these materials can be incorporated into a polymer used to make the article during processing, thereby providing a greater measure of permanence.
However, the incorporation of such ESD materials (ESD and/or antistatic agents) presents serious problems. For example, the high temperatures required for conventional processing of most polymers will often damage or destroy antistatic agents. Moreover, many ESD agents are not miscible with the matrix or base polymers in which they are used. These issues can lead to reduced moldability of the polymer, or blends containing the polymer, as the antistatic agents can migrate or diffuse to the surface during processing and deposit a coating on mold surfaces, possibly destroying the surface finish on the manufactured part being molded. In severe cases, the surface of the article becomes oily and marbleized. A large number of antistatic agents are also either cationic or anionic in nature. These agents tend to cause the degradation of plastics, particularly PVC, and result in discoloration or loss of physical properties. Additional problems which can occur with ESD agents include loss of their ESD capability due to evaporation, wear and rinsing, the development of undesirable odors, and the promotion of stress cracking or crazing on the surface of an article in contact with the article of manufacture. ESD agents may also be very sensitive to moisture, leading to reduced effectiveness in applications that expose the agent to water.
There are several examples of high molecular weight ESD agents in the prior art. In general, these additives have been high molecular weight polymers of ethylene oxide or similar materials like propylene oxide, epichlorohydrin, glycidyl ethers, and the like. It has been a requirement that these additives be high molecular weight materials to overcome the problems related to the migration, evaporation and/or the thermal stability of the ESD additive. However, these prior art ESD additives do not have a desired balance between electrical conductivity and acceptable low levels of extractable anions, in particular chloride, nitrate, phosphate, and sulfate, which in turn can cause any manufactured articles containing such ESD additives to have unacceptable properties for some end uses.
In addition, there are examples in the art of certain polyether thermoplastic urethanes that have good ESD properties, with and without the use of antistatic agents. However, polyether TPU compositions generally have poor properties including poor phase separation characteristics, difficulties achieving high molecular weights, inadequate compatibility with many other types of host polymers resulting in poor physical properties, and poor heat performance making them unsuitable for many applications. Polyester TPU compositions generally have better physical properties, but poor ESD properties. There is a need for TPU compositions that possess both the ESD properties of polyether TPU compositions and the physical properties of polyester TPU compositions.